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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
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Chapter14 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the
group and not to the CPU. Any unknown multicast traffic is flooded to the VLA N and sent to the CPU
until it becomes known.
Figure14-2 Second Host Joining a Multicast Group
Leaving a Multicast GroupThe router sends periodic multicast general queries and the switch forwards these queri es through all
ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to
receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch
forwards multicast group traffic to only those hosts listed in the forwarding table for that Layer2
multicast group.
When hosts want to leave a multicast group, they can either silently leave, or they can send a leave
message. When the switch receives a leave message from a host, it sends a group-specific query to
determine if any other devices connected to that interface are interested in traffic for the specific
multicast group. The switch then updates the forwarding table for that MAC group so that only those
hosts interested in receiving multicast traffic for the group are listed in the forwarding table. If the router
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
Table14-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
0100.5exx.xxxx IGMP 0
0100.5e01.0203 !IGMP 1, 2, 5
Forwarding
table
CPU
Server
Blade 1
Server
Blade 2
Server
Blade 3
Server
Blade 4
Router A
IGESM
Switching engine
1
0
2345
92422